This invention relates to color cathode ray tubes of the type having a four-corner suspended frameless shadow mask, and is especially concerned with proper suspension of the mask.
The conventional color selection electrode or "shadow mask" comprises a dished apertured mask which is welded to a rigid frame that imparts the necessary rigidity to the dished section. This type of mask-frame assembly is mounted in close adjacency to the faceplate of the cathode ray tube by a suspension system comprising three or four leaf springs. The springs are welded to the frame at selected points around the periphery. Because of the weight of the mask, it is necessary that the springs be relatively stiff; this stiffness results in the application of a load directed radially inwardly of up to four or five pounds to the mask-frame assembly. The distal ends of the springs are apertured to engage studs which project inwardly from the rearwardly extending flange of the tube faceplate. It is necessary that the mask-frame assembly be capable of being demounted and remounted with exact precision in relation to the faceplate several times; this demounting and mounting is required in the manufacturing process. Demounting is accomplished by depressing the springs to disengage the studs, and separating mask from panel usually by automatic machinery. In remounting, the shadow mask and the faceplate are brought into propinquity whereby the springs are caused to re-engage the studs.
The type of mask described; that is, one having a heavy, rigid frame which in turn provides the necessary rigidity to the dished, apertured mask, has significant disadvantages despite its proved commercial viability. The disadvantages include costliness, excessive mass, heavy weight, and the tendency to distort upon heating under the impact of the cathode ray tube electron beams. As a result of this distortion, it has been necessary to design elaborate and costly shadow mask suspension systems wherein bimetallic components provide the necessary "Q-spacing" to compensate for such heating. Q-spacing is defined as the required spacing between the shadow mask and the phosphor screen. The Q-spacing must be accurately maintained for proper registration of mask apertures with the associated phosphor stripes (or dots).
The problems inherent in a rigid mask design as described has led to the invention of a low cost, light-weight, non-self-rigid, torsionally flexible shadow mask, preferably of one-piece, frameless construction. A shadow mask of this type is disclosed in U.S. Pat. No. 4,100,451 to Palac, of common ownership herewith, wherein an approximately rectangular, flangeless curved faceplate supports on a concave inner surface thereof in a central region a phosphor screen comprising patterns of red-, blue-, and green-emissive phosphor triads. The low-mass, approximately retangular, non-self-rigid, torsionally flexible shadow mask has a central portion with the patterns of electron-transmissive apertures in registry with the patterns of phosphor triads. The mask has a rim portion providing substantial rigidity with respect to axes normal to the sides thereof, while providing for flexure of the mask with respect to its diagonals. The mask suspension system establishes a predetermined position of the mask relative to, and at a predetermined spacing from, the inner surface of the faceplate. The system includes four suspension means for mechanically coupling the mask directly to the corner portions of the faceplate. The suspension means are located one at each corner of the mask to permit the mask to flex about its diagonals and conform to the contour of the faceplate despite any twist-wise deformation thereof. By this means, the predetermined spacing between the mask and the faceplate inner surface is maintained.
A plan view of a shadow mask-face panel assembly wherein the mask is of the frameless type is shown by FIG. 1. The assembly 10 comprises a substantially rectangular shadow mask 12 having a dished perforate central section 14. Included in the assembly is a face panel 16 having a rearwardly extending flange 18.
Four identical suspension devices 20 provide for rigidly and stably suspending mask 12 in proper spatial relationship to the dished central section of face panel 16. As shown, one such device is located at each corner of the face panel 16 to provide for four-corner mounting of the shadow mask 12. This configuration is disclosed in the referent copending applications Ser. Nos. 024,272; 084,708; 101,959; and 144,719 and in referent U.S. Pat. Nos. 4,300,071 and 4,317,064. In consequence of the four-corner mounting system, the mask, which is low in cost but inherently lacking in self-rigidity due to its one-piece, frameless construction, is suspended with high rigidity derived from the rigidity of the glass face panel 16. The suspension system furnishes a mechanically rigid link between the flange of the face panel and the mask, yet permits the mask to be conveniently and repeatably demounted and precisely remounted with respect to the face panel.
Four studs 24 are shown as being affixed to the flange 18 of the face panel 16 on the associated diagonals 28 and 30. The studs 24 are arranged to extend substantially radially inwardly on diagonals 28 and 30 and indicated. Leaf springs 32 provide for detachably interconnecting the corners of mask 12 to the studs 24 extending from the flange 18 of the face panel 16. Each leaf spring is apertured for engagement with the associated tapered stud 24.
FIG. 2 shows in detail one of four corner sections of the face panel 16 and the rearwardly extending flange 18. Mask 12 has a rearwardly extending skirt 34 and an integral rim which extends radially outwardly. The mask suspension device 20 (notable as being one of four identical devices) comprises a stud 24 affixed to the face panel flange 18, and a leaf spring 32 for detachably interconnecting the corner of mask 12 shown to stud 24. Means for mounting spring 32 on mask 12 is shown in this embodiment as comprising a bracket 36 having two arms 38 and 40 for attachment to mask 12 and spring 32. Spring 32 is mounted on mask diagonal 30 and normal to the diagonal such that spring 32 extends rearwardly away from the dished central section of face panel 16. When spring 32 is deflected, its distal end 47 travels in an arc inwardly toward the faceplate center axis, all as disclosed and claimed in referent U.S. Pat. No. 4,300,071.
Because of the lack of the frame in a mask of the type described, the attachment of the springs 32 to the mask is by means of the relatively stiff brackets 36 which embrace the mask 12, and to which the mask 12 is welded. The suspension springs 32 in turn are welded to suitable areas of the brackets 36. The pressure exerted by the springs when the mask is in the mounted position, and when the mask is demounted and remounted, is redistributed to the mask by the brackets so that the non-self rigid mask is not distorted. The suspension system is designed to precisely fix and hold the predetermined spatial position of the mask as a whole relative to the faceplate against translational or rotational displacement, this in spite of any thermal expansion or contraction of the mask, frictional restraint during demounting and remounting, mechanical shocks, or force of gravity.
With reference again to FIG. 2, the bracket 36, shown as having two arms 38 and 40 for embracing the rearwardly extending skirt 34 of mask 12, is preferably comprised of cold-rolled steel having a thickness of about 60 mils. The bracket may be formed by well-known means such as stamping, coining, bending, shearing, punching, or piercing, etc. The bracket 36 is preferably welded to the skirt 34 of mask 12 by spot welding, indicated by weld points 44 in each of the arms 38 and 40.
The material of which the frameless mask 12 and the unitary skirt 34 to which the bracket 36 is welded, typically comprises a mild steel having a thickness of about six mils. In consequence, mask 12 is highly susceptible and yielding to distortive forces such as may originate in the mask suspension means. As has been noted, the brackets provide for redistributing the pressures exerted by the suspension springs.
The bracket themselves however may be a source of mask distortion. Any steel fabricating process, such as the stamping process by which the brackets may be shaped, is inherently incapable of producing an exact contour replica of a master in every piece formed. As a result, if one or more of the four brackets used for suspending the mask deviates appreciably from the desired contour, the thin, relatively flexible shadow mask, which is only about one-tenth the thickness of the bracket, will deform to conform to the contour of the thick, relatively inflexible bracket. Thermal processing of the cathode ray tube after the face-panel has been screened, such as heating the tube for frit sealing, will distort the mask by relieving the stress, and the mask apertures will be out of registry with the associated phosphor elements.
In U.S. Pat. No. 3,999,098 to Dougherty, assigned to the assignee of the present invention, there is disclosed means for suspending a frameless, torsionally flexible shadow mask in adjacency to a flangeless faceplate. The bracket includes a pair of diverging arms. The arms have wings bent out of the plane of the arms. The wings have provision for radial yield before welding, the provision indicated as being thinned-down sections. The wings are slightly overbent inwardly, and are allowed to "give" when the bracket is fixed on the mask. The thinned-down sections are intended to provide a low-force yield point for accommodation of manufacturing tolerances. It has been discovered, however, that the mounting geometry inherent in such a design is lacking in strength due to "puckering" of the mask material and subsequent permanent deformation as a result of application of certain forces to the bracket during normal production handling.